Objective: To test the utility of a single, weekly dose of iron in the treatment and prevention of anemia caused by iron deficiency in adolescent girls.Methodology: A randomized parallel clinical study was carried out with adolescent girls from 15 secondary schools in the state of Morelos, Mexico. Three study groups were formed in each secondary school: Two received different iron supplementation schemes: 1) a daily dose 2) a weekly dose; the third, the non-anememic group, did not receive any supplementation and was the control group. Initial and final samples of antecubital vein (venous blood) were taken, in order to establish the hemoglobin, hematocrit and cholesterol levels. The supplements were administered during a period of 16 weeks. Faeces were checked for parasitosis in order to make adjustments for same as possible confounding variables. Weight, height, waist and hip measurements were taken. Additionally, a survey was carried out which contemplated the following: dietary aspects (via a 24- hour dietary recall) socioeconomic aspects and questions relating to the clinical and psychological characteristics of menstruation, as well as the physical activity carried out by the adolescent girls.Results: 511 female secondary school students participated in the study. 31% were given 3 meals and 56% four and more; 59% of the girls claimed to have increased their daily intake of food in comparison to what they ate at primary school. 62% do not take any food to school from their home and 95% buy food at the school. 85% of the girls have a hereditary predisposition to obesity. The average basal levels of the girls were: weight, 46kg (SD 8.8); height, 151cm (SD 5.5); hemoglobin, 12.5g/dl (SD 0.6); hematocrit, 41% (SD 2.9). The hemoglobin levels in the daily supplementation group rose from 12.2 (SD 0.7) to 13.2g/dl (SD 1.0); in the weekly supplementation group very similar levels were found, from 12.2 (0.7) to 13.1g/dl (SD 1.0) and in the control group there was a rise of 13.3 (SD 0.4) to 13.6g/dl (DE 1.2). In the three groups, the differences were statistically significant (P<0.5).Conclusions: The characteristics of the sample make it possible for the results to be extrapolated to similar sectors of the population in other communities. After 16 weeks of supplementation, there was a similar type of behaviour in the two supplementation groups, which leads to the conclusion that the single weekly dose is as effective as the daily iron dose in the prevention and correction of iron deficiency in adolescent girls. Progress was made on knowing how to prevent iron deficiency and anemia caused by iron deficiency in adolescent girls, through the use of a single weekly dose which reduces costs, increases compliance to treatment and improves the absorption of iron.